This invention relates to refrigeration for stationary and transportable compartments or containers used to prevent deterioration or spoilage of farm produce, food products and other perishable materials in either solid or liquid form. More particularly, the invention involves refrigeration apparatus utilizing solid carbon dioxide (CO.sub.2) as the refrigerant and the method of charging pressurizable apparatus with liquid CO.sub.2 at elevated pressure to provide a pool of liquid CO.sub.2 therein and converting the liquid pool to solid cake or block form.
In spite of the extensive use of mechanical refrigeration systems with storage rooms and vehicles designed for perishable goods, there has been a growing realization that refrigeration from CO.sub.2 offers several advantages over mechanical refrigeration. For instance, CO.sub.2 refrigeration avoids the danger of unexpected mechanical failure, requires lower capital and maintenance costs, and eliminates dependence on a fuel subject to large cost variations.
A simple and effective CO.sub.2 refrigeration system which has gained increasing acceptance for boxcars is disclosed and claimed in the present inventor's U.S. Pat. No. 3,561,226. The patented system which has also been applicable to trucks involves spraying pressurized liquid CO.sub.2 into an atmospheric box whereupon the sprayed liquid CO.sub.2 becomes flakes. The CO.sub.2 flakes accumulate in the box much like snow forms a mass or blanket on the ground. As known, such masses of flakes have very low density. Obviously, it would be advantageous to store in the refrigeration system CO.sub.2 as a solid of high density so that more refrigeration could be stored per unit of volume in the truck or other container requiring refrigeration. The aforesaid patented system of filling an atmospheric cold box with CO.sub.2 flakes depends upon the flashing or vaporization of roughly half of the liquid CO.sub.2 which is sprayed. Hence, the resulting CO.sub.2 vapor which contributes little refrigeration to the boxcar or truck before escaping therefrom into the atmosphere is largely an economic loss.
An improved CO.sub.2 refrigeration system for preserving perishable goods and liquids in containers has now been designed to optimize its refrigeration capacity and uniformity as well as to increase its economic attractiveness.
A principal object of this invention is to provide a CO.sub.2 refrigeration system in which a solid cake of CO.sub.2 is formed as the refrigerant.
A further object is to recover CO.sub.2 vapor evolved when charging the refrigeration system with solid CO.sub.2 or when draining the system of residual CO.sub.2 prior to a shutdown or disuse period.
Other features and advantages of the invention will be apparent from the description which follows.